1. Field of the Invention
The invention relates generally to switches used to activate circuitry housed in a tubular device and in particular to a circumferential switch to activate circuitry in a tubular housed electronic device and a method for making the circumferential switch.
2. Description of the Related Art
Most switches on barcode wands and other tubular devices are activated by a single button, key, or some type of actuator that is positioned at a particular location on the scanner body or tube. To activate the switch, as a bar code is scanned, requires an operator to always hold the device at the same position or orientation, and with some designs at the same vertical location. Such a switch does not take into account anatomic variables, and is not as convenient as a circumferential switch.
As its name implies, a circumferential switch wraps around the entire surface of the tubular device, i.e., the circumference, and is positioned in the location typically used to hold the device. The circumferential nature of the switch allows activation of the switch at any rotation, orientation, or angle as the device is used for barcode scanning. The switch is engaged by simply picking up the device like a pencil and lightly squeezing on the switch area.
One circumferential barcode wand switch 100, hereinafter "barcode switch 100" is illustrated in FIG. 1. Barcode switch 100 is available from Welch Allen of Skeneatles Falls, New York. Housing 104 is a cylindrical metal tube that houses bar code scanning circuitry mounted on board 106. A two membrane circuit 110 is mounted to the outer circumference 104A of housing 104 with adhesive on the non circuit side of two membrane circuit 110. Two membrane circuit 110 is mounted near tip 102 of the barcode wand. A rubber cover 103 surrounds a two membrane circuit 110.
Two membrane circuit 110 of barcode switch 100 is made of two clear, flexible, plastic membranes. A pattern is printed with silver conductive ink 212 (FIG. 2) on a surface of each membrane. The membranes are laminated together such that the printed patterns face each other, but are separated from each other by a very small air pocket, referred to as air gap 101 (FIG. 1), that is formed between the two membranes.
A tab 111 (FIGS. 1 and 2) has a first conductor 220 connected to the printed pattern on one membrane and a second conductor 221 connected to the printed pattern on the other membrane. Conductors 220, 221 are insulated from each other so that they do not make electrical contact. Tab 111 extends from the two membranes though a square hole 105 (FIG. 1). As tab 111 extends through hole 105, tab 111 makes a 180 degree bend 107, and is connected to board 106 by electrical connector 108.
When the operator squeezes bar code switch 100 at any point, the air in air gap 101 is compressed and the two printed patterns make electrical contact. The electrical contact is maintained as long as the operator maintains pressure on bar code switch 100. When the pressure is released, bar code switch 100 returns to the original open position.
Unfortunately, while circumferential bar code switch 100 is easier to use than other single location switches, the functionality and durability of bar code switch 100 is poor. The problems that exist are primarily associated with the two printed circuits on the membranes. The two layers of silver conductive ink 212 (FIG. 2) are made up of tiny particles of silver suspended in a printable ink, which when shorted together are neither durable nor reliable. Furthermore, if during assembly, or actual use, the silver conductive ink layers are kinked, scratched, folded, impacted or otherwise harmed, the life expectancy is reduced and may even cause immediate failure of switch 100.
Most failures occur at 180 degree bend 107. If tab 111 is not aligned precisely in square hole 105, or if tab 111 is not a precise length, tab 111 contacts the long flat sharp edges of square hole 105. If membrane circuit 110 is affixed to the metal housing twisted, relative to square hole 105, tab 111 enters square hole 105 at an improper angle. Consequently, the sides and even the conductive ink of tab 111 may be cut by contact with the edges of square hole 105. Thus, production yields are poor, if close attention is not paid to all of these problems.
Unfortunately, a failure may not show up in production inspection, because the cut or damage to tab 111 during assembly may not be sufficient to cause a failure. However, upon use of the device, tab 111 is damaged further and a failure occurs.
If the device is dropped, since square hole 105 is near tip 102, the impact of the tip with an object can result in tab 111 contacting the sharp edge of square hole. Over time, contact between an edge of square hole 105 and tab 111 causes tab 111 to fail. Generally, contact between square hole 105 and tab 111 cuts or wears the conductive ink off, or cuts through tab 111. All of this adds up to a high failure rate of barcode switch 100 both in production and in actual use. Hence, while a circumferential switch is ergonomically preferable, the reliability of such a switch limits its acceptance, and increases manufacturing costs, which in turn increases the cost to the end user.